gradle-plugin: fix docs and tests for Groovy DSL

[petertrr]

What's done:

• Update build.gradle in examples
• Add functional test for Groovy DSL with explicit inputs
• Fix docs for diktatExtension.output: reporter treats any non-empty string as a file name, there are no special values
• Remove dependsOn relation between test and functionalTest, use shouldRunAfter instead

This pull request closes #1422

[codecov]

Codecov Report

Merging #1428 (62723a1) into master (0cd6d17) will decrease coverage by 0.14%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1428      +/-   ##
============================================
- Coverage     83.34%   83.19%   -0.15%     
+ Complexity     2562     2555       -7     
============================================
  Files           107      107              
  Lines          7612     7612              
  Branches       2103     2103              
============================================
- Hits           6344     6333      -11     
- Misses          376      388      +12     
+ Partials        892      891       -1     

Flag	Coverage Δ
unittests	83.19% <ø> (-0.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...n/org/cqfn/diktat/plugin/gradle/DiktatExtension.kt	100.00% <ø> (ø)
...qfn/diktat/plugin/gradle/DiktatJavaExecTaskBase.kt	74.10% <0.00%> (-9.83%)	⬇️

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[github-actions]

JUnit Tests (macOS, EnricoMi/publish-unit-test-result-action@v1)

1 271 tests   1 256 ✔️  1m 26s ⏱️
   156 suites       15 💤
   156 files           0 ❌

Results for commit 98a2b50.

♻️ This comment has been updated with latest results.

[github-actions]

JUnit Tests (Windows, EnricoMi/publish-unit-test-result-action@v1)

1 271 tests   1 256 ✔️  1m 0s ⏱️
   156 suites       15 💤
   156 files           0 ❌

Results for commit 98a2b50.

♻️ This comment has been updated with latest results.